1. Field of the Invention
This invention relates generally to mechanical dampers for cantilevered structures and, in particular, it relates to a tuned-mass strand-impact damper which mitigates vibrations in cantilevered traffic signal structures.
2. Description of the Prior Art
The fatigue resistance of traffic and sign structures across the U.S. in recent years has become a concern. A number of sign and signal fatigue failures have occurred as a result of the large number of stress cycles caused by wind-induced vibrations. Sign and signal structures are generally light and have low inherent damping. Traffic signal structures, in particular, have a low fundamental period of vibration, contributing to aeroelastic phenomenon such as galloping or vortex shedding. Galloping can cause large vertical displacements in relatively mild wind.
Two traffic signal structures recently collapsed in Wyoming. The collapse was the result of a fracture at the connection between the cantilever signal light support arm (mast arm) and the pole connected to the foundation. The Wyoming Department of Transportation (WYDOT) has confirmed (through an inspection of the failed connections) that the traffic pole failed at the toe of the welds as a result of fatigue cracking. The structures did not fail under an extreme-event wind, and the fatigue crack growth was most likely caused by vibrations at lower wind speeds. Recent research has indicated that these vibrations may be caused by galloping of the cantilever pole during wind speeds in the range of ten (10 mph) miles per hour to thirty (30 mph) miles per hour. These wind speeds were also found to cause vibrations with cantilever tip amplitudes of eight (8 in.) inches to eighteen (18 in.) inches in a full-scale test specimen (with a forty-eight (48 ft.) foot cantilever) subjected to actual wind conditions of ten (10 mph) miles per hour to thirty (30 mph) miles per hour.
WYDOT visual inspections of approximately eight hundred and forty (840) poles indicated that roughly one-third (⅓) of the poles inspected have fatigue cracks ranging in length from one-quarter (¼ in.) inch to twenty (20 in.) inches around the box connection between the pole and mast arm. Because visual inspection only indicates cracks that have propagated to the surface, the damage is likely more significant than these numbers suggest.
In the past, a tuned-mass and impact damper combination was created by supporting a mass with a short section of prestressing strand with the prestressing strand acting as a spring for the mass. The length of strand was adjusted so that the natural frequency of the damper was close to that of the in-plane mode of the structure. The impact mass was centered between horizontally positioned steel impact plates on the top and bottom and vertically positioned threaded rods on the sides. The distance between the impact plates was adjusted to maximize the damping provided.
Unfortunately, with the horizontally positioned impact plates and the vertically positioned threaded rods, the tuned-mass and impact damper combination of the prior art did not provide equal stiffness in all directions thereby adversely affecting the dampening of the structure in certain directions. Furthermore, tuning this tuned-mass and impact damper combination was very difficult and matching the natural frequency of the structure was extremely difficult.